Traditional ultrasound Doppler fetal monitor uses analog demodulation to analyze fetal heart rate. Because the demodulated signal in this mode is a single channel signal, vector analysis can not be carried out. Therefore, in the practical application process, there is repeated recognition of positive and negative movement of mitral valve, resulting in the phenomenon of heart rate doubling. In this research, we designed and implemented a wireless ultrasound Doppler fetal monitoring system based on digital demodulation mode. The system uses the time domain windowing method to demodulate the ultrasonic echo signal, and analyzes the heart rate according to the vector velocity curve. The system can effectively avoid the phenomenon of heart rate doubling. At the same time, the system uses the data wireless transmission scheme between the host and probes, which greatly facilitates the use of operators. According to the results of standard fetal heart phantom test, the system can effectively solve the defect of heart rate doubling existing in traditional equipment, and realize stable, reliable and convenient fetal monitoring.